Plastics are low weight and relatively tough materials suitable for making various kinds of products for use in our daily life, and plastic-related craft and plastic industry are always important to the modern society. A general injection-molding machine mainly includes two units, namely, an injection mechanism for injecting thermal plastic material and a forming mechanism for opening and closing a mold. The mold generally includes a fixed part and a movable part. Plastic material fed into a material barrel of the injection mechanism enters into an injection barrel via a valve on the material barrel, and is forced forward by a rotating spiral band. At this point, plastic granules are forced to contact with the wall of a heating barrel and heated, molten, and softened. The plastic material is further forced forward in a spiral lead and injected into a mold cavity of a closing mold. A movable clamping cap of the mold is then moved to open or close the mold.
The movement of the movable part of the mold to close the mold is also referred to as clamping mold. The pressure or force at clamping the mold has important influence on the precision of the molded plastic material. Moreover, uneven distribution of the force at clamping the mold would result in bias of the molded plastic material. Therefore, it is necessary to measure the force at clamping the mold and use the measured value as a basis to adjust the clamping force.
The plastic injection-molding machine or die-casting machine developed in the early stage, either a vertical type or a horizontal type as illustrated in the accompanying drawings, typically includes four tie bars. The movable clamping cap is slidably mounted on the four tie bars to move relative to a fixed clamping cap (not shown) and thereby opens or closes the mold. In the process of clamping the mold on the forming machine, the movable clamping cap on the four tie bars applies pressure on the fixed part of the mold, and the applied pressure is axially fed back to the tie bars. The total pressure fed back to all the four tie bars is defined as the clamping force. The higher the pressure applied by the movable clamping cap is, the higher the clamping force is, that is, the higher the axial force fed back to the tie bars is. In other words, on the basis of the same molding material, the higher the pressure applied by the movable clamping cap is, the larger the axial stretch of the tie bars is.
Please refer to FIG. 1 and FIG. 1A that is an enlarged view of the circled area of FIG. 1. A conventional way to measure the clamping force is to provide a conventional meter 1 near a rear end of each tie bar 41 of a forming machine 4. When the tie bar 41 has any minor change in its overall length, that is, has any axial stretch, during the process of clamping the mold to touch a probe 11 of the meter 1, the probe 11 would contact with and press against a spring inside the meter 1, so that the amount of axial stretch of the tie bar 41 may be read from a pointer 12 of the meter 1. The force axially applied to the tie bar 41, that is, the clamping force, may then be calculated from a specific formula using the reading of the meter 1. This way of measuring the clamping force has the following disadvantages:                1. The conventional meter has low accuracy, and the spring inside the meter is subject to elastic fatigue after being used over a long time.        2. An operator has to visually observe the reading of axial stretch amount of the tie bar on the conventional meter. Errors might be caused due to different observation angle and personal subjective judgment or negligence to result in inaccurate data.        
FIG. 2 shows another conventional strain meter 2 developed for measuring the clamping force, and FIG. 2A is an enlarged view of the circled area of FIG. 2. Please refer to FIGS. 2 and 2A at the same time. The strain meter 2 mainly includes a sensing element 21 having two signal lines 22 extended therefrom. The two signal lines 22 are electrically connected to a monitor 23, so that a signal from the sensing element 21 may be processed by an operation unit and amplified by an amplifier unit, and finally displayed on the monitor 23. This type of strain meter 2 is flatly adhered to an outer surface of the tie bar 41 of the forming machine 4. When the movable clamping cap is slid on the tie bars 41 relative to the fixed clamping cap to open or close the mold, the sensing element 21 is able to detect a change in the electric resistance of the tie bars 41 due to the axial stretch thereof, and thereby measures the clamping force at the time the mold is closed. However, this type of strain meter 2 has the following disadvantages:                1. The sensing element is removably adhered to the outer surface of the tie bar. When the sensing element is released from the tie bar after use, it is damaged and the strain meter must be discarded. It is apparently not economical to discard the high precision and costly strain meter when the same has been used for only one time.        2. The sensing elements are not easily adhered to the tie bars at uniform angle and tightness, and would therefore have adverse influence on the measurements.        3. It is troublesome and time-consuming to accurately adhere the sensing elements to the tie bars at uniform angle and tightness.        4. The signal from the strain meter must be amplified with the amplifier, and the amplifier must be separately provided to inevitably interfere with the layout of the working site. Moreover, complicate procedures are involved in wiring the amplifier and related parts, and the amplifier could not be conveniently moved once it is fixed to a certain position.        